…Allah’s Messenger (pbuhap) was in salat[prayer] and to his side was al-Husayn b. `Ali. So Allah’s Messenger (pbuhap) did takbir and al-Husayn (pbuh) did not respond with takbir. Then Allah’s Messenger (pbuhap) did takbir and al-Husayn did not respond with takbir.
So Allah’s Messenger (pbuhap) did not cease from doing takbir and al-Husayn (pbuh) did not take up the takbir so he did not respond until he completed seven takbirat. So al-Husayn (pbuh) responded (with) the takbir in the seventh.
Then Abu `Abdillah (pbuh) said: So it turned into the Sunna.